Linux Checksum Calculation Logic

^API | Comments |
| skb_checksum_help |  |
| ip_fast_csum | |
| csum_fold | |
| skb_checksum | |
| csum_tcpudp_magic | |
| nf_ip_checksum |
| csum_tcpudp_nofold |
| csum_replace4 |
| csum_replace2 |
| nf_nat_ipv4_csum_recalc |
| csum_tcpudp_magic |


skb->csum_start = skb_transport_header(skb) - skb->head;

skb->csum_offset = offsetof(struct tcphdr, check);

**TSO** is a hardware feature whereas **GSO** is a software feature.

In the sense, TSO needs "the device" to break the frame into MTU sized segments.
BUT, GSO is a strategy followed by kernel to avoid processing smaller packets throughout the stack to improve the porformance

====== Linux Checksum offloading ======
 
  - For bridging and non-udp/tcp packet, no so called offloading support in tx path
  - For Fragmented packets, no checksum offloading support in tx path
  - For UDP/TCP routing packet (incremental), no checksum offloading support in tx path. at the same time, no ip_summed flag change
  - Only Linux local output UDP/TCP’s checksum can be offloaded in tx path 
  - For L2TP related tunnels, only inner UDP/TCP can be offloaded, outer UDP/TCP is done by linux itself in tx path.

Below variables in skb is very important for checksum offloading:
  - ip_summed (CHECKSUM_PARTIAL)
  - encapsulation
  - inner_network_header/network_header
  - inner_transport_header/transport_header


====== Linux Checksum ======
<file>
union {

    __wsum                       csum;
    struct {
        __u16  csum_start;
        __u16  csum_offset;
   };
}; 
</file>


| Variables/Direction | skb->ip_summed | skb->csum or csum_start/offset |
^ Input | CHECKSUM_NONE 0  | not valid | SW need do L4 payload + pseudo header verification  |
| | CHECKSUM_UNNECESSARY 1 | not valid | HW already do L4 payload + pseudo header verification  |
| | CHECKSUM_COMPLETE 2 | csum valid (stored with L4 payload checksum) | SW need to do skb->csum + pseudo header verification |
^ Output | CHECKSUM_NONE 0 | not valid | SW already do necessary L4 checksum calculation  |
| | CHECKSUM_PARTIAL 3 | csum_start/offset valid | SW already do pseudo header and HW need to further calculate its payload based on   |